Generally, interconnections between electronic components can be classified into the two broad categories of "relatively permanent" and "readily demountable".
An example of a "relatively permanent" connection is a solder joint. Once two electronic components are soldered to one another, a process of unsoldering must be used to separate the components. A wire bond, such as between a semiconductor die and inner leads of a semiconductor package (or inner ends of leadframe fingers) is another example of a "relatively permanent" connection.
An example of a "readily demountable" connection is rigid pins of one electronic component being received by resilient socket elements of another electronic component. The socket elements exert a contact force (pressure) on the pins in an amount sufficient to ensure a reliable electrical connection therebetween. Interconnection elements intended to make pressure contact with an electronic component are referred to herein as "springs" or "spring elements" or "spring contacts".
Prior art techniques for making spring elements generally involve stamping (punching) or etching a "monolithic" spring material, such as phosphor bronze or beryllium copper or steel or a nickel-iron-cobalt (e.g., kovar) alloy, to form individual spring elements, shaping the spring elements to have a spring shape (e.g., arcuate, etc.), optionally plating the spring elements with a good contact material (e.g., a noble metal such as gold, which will exhibit low contact resistance when contacting a like material), and molding a plurality of such shaped, plated spring elements into a linear, a peripheral or an array pattern. When plating gold onto the aforementioned materials, sometimes a thin (for example, 30-50 microinches) barrier layer of nickel is appropriate.
Generally, a certain minimum contact force is desired to effect reliable pressure contact to electronic components (e.g., to terminals on electronic components). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per contact) may be desired to ensure that a reliable electrical connection is made to a terminal of an electronic component which may be contaminated with films on its surface, or which has corrosion or oxidation products on its surface. The minimum contact force required of each spring element typically demands either that the yield strength of the spring material or that the size of the spring element are increased. However, generally, the higher the yield strength of a material, the more difficult it will be to work with (e.g., punch, bend, etc.). And the desire to make springs smaller essentially rules out making them larger in cross-section to achieve greater contact forces.
The PARENT CASE (U.S. patent application Ser. No. 08/452,255) describes the fabrication of resilient contact structures (spring elements) as composite interconnection elements by mounting a free-standing wire stem (elongate element) on a terminal of an electronic component, shaping the wire stem, and overcoating the free-standing wire stem to impart the desired resiliency to the resulting free-standing spring element. In the case of electronic components which are delicate and/or expensive, faulty fabrication of a composite interconnection element directly on the surface of the electronic component can require, at best, reworking the faulty interconnection element(s) or, at worst, discarding the electronic component. As mentioned in the PARENT CASE, composite interconnection elements can be fabricated (e.g., bond, shape, overcoat) on sacrificial substrates, then either singulated therefrom or gang-transferred (mounted en masse) to an electronic component.